This disclosure relates generally to communication and, more specifically, to techniques for facilitating communications between networked nodes (e.g., in an industrial control system).
Industrial control systems are used in industrial processes to control pressures, temperatures, mass transfers, electrical contact closures, and other process parameters. Control systems may include many input/output (I/O) devices (e.g., temperature sensors, pressure sensors, pressure regulators, electrical relay, and/or solenoid actuations, and other similar devices) positioned at various locations in process equipment being controlled. Modern process control systems may also include one or more programmable logic controllers (PLCs) for controlling various output devices in the system, based on data collected from input devices and based on instructions programmed into the PLCs.
Typically, I/O devices at one or more locations communicate with I/O devices at other locations in order for a control system to control a process. In particular, input devices in one location may transfer their data to output devices in one or more other locations to facilitate control of a process. In general, an industrial control system requires a data transfer arrangement to facilitate communication and data sharing between control system devices.
Some industrial processes may require very precise control. A communication failure between elements of an industrial control system may upset an entire process and have dire consequences. For example, failure of process control communications in a chemical manufacturing process may result in the production of an entirely different chemical than what was intended. In this case, a communications failure in an industrial control system may require that a process being controlled be aborted or at least suspended until communication is reestablished. Also, failure of communication and control in some processes may pose very serious safety risks. In general, communication systems employed in industrial control systems should be robust.
Data communication lines required between various elements of an industrial control system commonly traverses harsh environments or areas of high activity. In harsh environments and areas of high activity there is a constant danger of damage to data communication lines resulting in a loss of data communication in a control system. Also, conditions in some areas of an industrial process may interfere with certain types of transmissions. For example, electrical noise in certain areas of a plant may interfere with electrical signal transmissions.
To deal with electrical noise, data may be converted to optical signals for transmission between various input and output devices via optical fibers. Various nodes or locations in an industrial control network may be connected for data communication in a ring that allows communications in both directions around the ring with data shared at each node. In a closed ring configuration, a single break in a link between nodes of the ring does not cause a loss of data transfer since communication is still possible in the opposite direction from the link break location. However, a closed communication ring may suffer from continuous data repetition around the ring which interferes with new data transfer over the ring.
One way to solve the problem of ensuring communications in the event of a link failure between nodes along a communication pathway in an industrial process control system is to provide two communication pathways between the various nodes of the system. In the event of a failure on a primary communication pathway, communications may still be maintained along a secondary communication pathway. Since uncontrolled data messages in a multi-pathway network can give rise to repetitive circling of the same data around the ring (where new data is prevented from accessing the network), great care must be taken to prevent this situation.
Still further complications arise due to the nature of data communications in an industrial control system. While some communications of data may be relatively continuous in some process control applications, data communications may be infrequent in other systems. In situations in which long gaps appear between data, a communication failure may not be readily apparent. That is, a system that monitors for communication failures may mistake a communication fault for a gap in data or may mistake a gap in data for a communication fault. As such, detecting a failure in a communication process in an industrial control system may be difficult.
U.S. Pat. No. 6,307,652, which is hereby incorporated herein by reference in its entirety for all purposes, discloses a control unit that is included in a duplex optical communication ring that operates, at least in some respects, as a master unit to control aspects of communication within the ring. The control unit initially blocks data in both paths of the ring, i.e., in both transmit and receive directions of transmit and receive optical ports of the control unit. Data units are included in the ring and are operable both to interface respective control or data acquisition system devices to the ring and also to control aspects of communication within the ring, i.e., as slaves to the control unit in at least some respects. Responsive to detection of a break in a fiber link, the system moves the location of the blocking. That is, if a data unit detects a break, the data unit responsively blocks data at the location of the break and signals the master unit to stop blocking the passage of communications in an alternate pathway.